Artificial disc device

ABSTRACT

Artificial disc devices are disclosed that restore correct anatomical intervertebral spacing for damaged discs while maintaining a substantially normal range of biomechanical movement for the vertebrae between which they are implanted. The disc devices include center bearing and outer or annular bearing portions with the center bearing portion including generally axially extending locating surfaces which cooperate with the facing vertebral surfaces to resist migration. The outer bearing portion is for load bearing or load sharing with the center bearing portion and includes surfaces that extend radially toward the periphery of the vertebrae so that subsidence about the center bearing portion is minimized. Alternate forms of the disc devices include one with an axially enlarged center ball bearing having an annular ring bearing extending thereabout and another having upper and lower plate members with a central bumper member and a surrounding resilient annular member therebetween.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/282,620, filed Oct. 29, 2002, entitled “Artificial Disc Device,”which claims priority to U.S. Provisional Application No. 60/382,758,filed May 23, 2002, entitled “Artificial Intervertebral Disc Device,”and which are incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Artificial disc technology has been employed to correct damaged spinaldiscs for relieving back pain and restoring or maintainingintervertebral spacing while attempting to minimize their constrainingeffects on the normal biomechanical movement of the spine. Two types ofartificial discs have generally been employed: the artificial total discwhich is designed to substitute for the entire disc, i.e. the annulus,nucleus and possibly the end plates as well; and the artificial nucleuswhere only the nucleus is replaced with the annulus and end platesremaining intact. The disc of the present invention is not intended tobe limited to one or the other of the above types.

A number of prior artificial disc devices include upper and lowermembers that are rigidly fixed to the adjacent upper and lowervertebrae. These fixed members sandwich a bearing therebetween alongwhich they can slide to allow for relative movement between the adjacentvertebrae, see, e.g. U.S. patent application Publication 2002/0035400.However, devices such as these usually require special surface materialsand/or surface treatments that allow for bone ingrowth for fixing themembers to the vertebrae. Moreover, these devices have had problems withmigration where the intermediate bearing body shifts out from betweenthe vertebrae, and thus generally require more complex shapes to formstops for resisting such disc shifting.

In a relatively early approach, a stainless steel ball was employed inthe damaged disc area. The ball approach, while effective to provide agood range of motion, tended to create subsidence problems. Over time,the ball would crush into the end plates as loading was fairlyconcentrated over a small surface on the ball in engagement with theplates. In other words, since these ball implants were not of a sizethat enabled the load of the spine to be distributed evenly thereacross,the end plates tended to subside or fall around the ball.

There also has been focus on simply replacing the nucleus with a gelledsubstance either injected directly in the disc or provided in pouches toattempt to reinflate the annulus and provide for load bearing. However,these approaches are limited in their use to patients who have asubstantially undamaged disc annulus.

Accordingly, there is a need for an artificial disc that does notsignificantly inhibit spine movement while still providing the loadbearing and spacer functions akin to that of a normal, healthy spinaldisc.

SUMMARY OF THE INVENTION

In accordance with one form of the present invention, an artificial discdevice is provided including a central, enlarged bearing portion and anouter, annular bearing portion generally extending about the centralbearing portion and allowing for movement therebetween. The inner orcentral, enlarged bearing portion preferably has a body including upperand lower arcuate surfaces or surface portions that can shift relativeto the outer bearing portion as well as with respect to the confrontingsurfaces of the spine, such as the end plates of the vertebrae. In thisregard, the arcuate surfaces are not rigidly fixed to the vertebrae andare curved so as to allow the upper and lower vertebrae to shift withrespect to each other such as when the spine is bent from side to sideor front to back and twists or turns. At the same time, the enlargedcentral bearing portion can engage in small indentations in therespective vertebral surfaces that keeps the central bearing in arelative locked position thereby preventing lateral shifting withrespect to the vertebrae so that the implant does not migrate despitethe shifting vertebrae above and below these bearing surfaces. Thus, theenlarged central bearing portion locates the artificial disc devicerelative to the vertebrae.

The main body of the central bearing or bearing portion or bearingassembly including the arcuate bearing surfaces thereof can be a hardmetallic material or alloy for load bearing purposes. Alternatively, ahard plastic could be employed to provide the central bearing portionwith resiliency under compressive loading. For shock absorption, thebearing body may be provided with a hollow core or one that is liquid orgel filled or filled with other elastic material. To vary the give orcompressibility of the central bearing body, the size of the core couldbe enlarged or decreased accordingly, or the modulus of elasticity ofthe body material can be varied.

In one preferred form, the outer bearing portion has a body thatincludes radially inner surfaces adjacent the arcuate surfaces adaptedor configured for allowing relative movement therebetween. The outerbearing shares the compressive loading generated between the vertebraevia upper and lower bearing surfaces or surface portions thereof so thatthe load is better distributed across the present artificial disc deviceminimizing localized forces thereon. With the provision of the outerbearing, the present device is well suited to avoid subsidence problemsas could occur in prior devices having highly localized loading thereon.

The outer bearing or bearing assembly also may be constructed to provideimproved shock absorption capabilities such as with an inner portion ofthe body that is softer than the harder outer portion. For example, anelastomeric layer of material can be employed between attached upper andlower bearing plates of the outer bearing, or the core layer of anannular portion and/or an inner ball bearing portion of the outerbearing can be of elastomeric or liquid gelled material. Manifestly,material combinations can also be employed to achieve desired shockabsorption proportions. The outer bearing can further include acompression limiter so as to maintain proper tolerances between theouter bearing inner surfaces and the inner bearing surfaces inconfronting relation therewith as the outer bearing is loaded. In thismanner, the inner bearing maintains its freedom of movement despite thecompressive loading that is being borne by the outer bearing, as will bedescribed more fully hereinafter.

In one form, the artificial disc includes a central ball as theenlarged, central bearing portion with an annular body of the outerbearing extending thereabout. The upper and lower load bearing surfacesor surface portions of the outer bearing body preferably do not projectaxially as far toward the upper and lower vertebrae as the ball surfaceportions such as by having a larger radius of curvature than the radiusof the ball. In other words, the load bearing surface portions have amore gradual curvature than the center bearing surface portions or forthat matter they can have a flat configuration. This allows the enlargedball to seat in the indents in the end plates for positioning theartificial disc securely between the vertebrae while the annular body isalso effective in taking up the compressive loading between the upperand lower vertebrae.

In another form, the central bearing portion includes a pair ofgenerally dome-shaped shell members that ride on a generally sphericalinner bearing portion integral with the outer bearing portion forsliding thereover. In this regard, the inner bearing portion isintegrally connected to the outer bearing portion via a circumferentialweb wall. The domes or shells are sized relative to the inner sphericalbearing portion so that there are gap spaces between the peripheraledges of the domes and the web wall. The web wall positions the outer,annular load bearing portion such that interference with shifting of thedomes on the central spherical bearing portion is minimized.Alternatively, snap-fitting the domes in place over the inner ballbearing portion could be employed; however, the above describedloose-fitting construction is preferred to minimize binding of the domeshells under compressive load forces. In this manner, the domes canreadily slide on the inner ball portion and, at the same time, thevertebral end plates or other vertebral surfaces in engagement with thearcuate surfaces of the domes can also shift with respect thereto toprovide a bi-polar device with two interfaces that shift with respect toeach other.

By having this bi-polar artificial disc construction, the stress andwear that would otherwise occur in either of the interfaces is decreasedas one bearing interface can be shifting when the load on the otherbecomes too great. Lubricant can be provided between the dome shells andthe inner bearing portion to reduce friction and wear therebetween. Aseal ring attached adjacent or at the end edge of the shells for beingcarried therewith minimizes lubrication leakage while allowing theshells to slide over the spherical surface of the inner bearing portionin a low friction manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are directed to various views of one form of an artificialdisc implant device in accordance with the present invention showing anenlarged spherical central bearing and an outer annular bearing;

FIGS. 1F-1H and 1J are directed to various views of a disc deviceslightly modified over that shown in FIGS. 1A-1E to better conform tothe vertebrae;

FIGS. 2A-2D are directed to various views of the artificial disc deviceof FIGS. 1A-1E as implanted between adjacent upper and lower vertebrae;

FIGS. 3A-3D are directed to various views of an alternative artificialdisc in accordance with the present invention showing a pair of domeshells that ride on an inner, spherical bearing portion integral withthe outer annular bearing portion;

FIGS. 4A-4D are directed to various views of the artificial disc deviceof FIGS. 3A-3D implanted between upper and lower vertebrae;

FIGS. 5A-5E are directed to various views of an artificial disc devicesimilar to that shown in FIGS. 3A-3D except having a circumferentialgroove extending about the periphery of the outer bearing portion;

FIGS. 6A-6F are directed to various views of another artificial discdevice in accordance with the present invention showing a pair of outer,annular bearings that fit about an enlarged, central spherical bearing;

FIGS. 7A-7C are directed to various views of an alternative constructionof the central bearing showing opposing dome shells, one having acentral post projection and the other having a central hub;

FIGS. 8A-8E are directed to various views of an artificial disc deviceincluding the dome shells of FIGS. 7A-7C projecting into an openingformed in the inner bearing portion;

FIG. 9 is a cross-sectional view of an alternate form of the artificialdisc device of FIGS. 8A-8E showing a pair of inner bearing rings onwhich the respective dome shells ride with a cushion web walltherebetween.

FIGS. 10A-10D are directed to various views of another alternativeartificial disc device having an axially enlarged central bearing memberand an outer, annular bearing member showing an hour-glass configurationfor the central-bearing member and an apertured body of the outerbearing member;

FIGS. 10E-10H are directed to a modified version of the disc device ofFIGS. 10A-10D showing different sizes of through apertures formed in theouter bearing member;

FIGS. 11A-11H are directed to various views of alternative artificialdisc devices showing upper and lower bearing members and a load bearingmember therebetween; and

FIGS. 12A-12I are directed to various views of an alternative discdevice showing upper and lower plate members, and an annular loadbearing member and a plug member therebetween.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referencing FIGS. 1A-1E, an artificial disc device 10 is shown whichincludes an enlarged, central bearing portion 12, and a substantiallyannular, outer bearing portion 14 having a through opening 15 in whichthe central bearing portion 12 is disposed. Herein, preferred shapes,configurations and material selections for the inner and outer bearingportions are set forth. However, in each case, these selections are notmeant to be limiting as other selections that serve the purpose of thedisc implant described herein are also contemplated. Likewise, severalembodiments are disclosed that have structural features that can beimplemented substantially interchangeably among the disc implants.

In the form illustrated in FIGS. 1A-1E, the central bearing 12 has anaxially enlarged body 13 relative to the outer bearing 14 so that itgenerally includes arcuate surface portions 16 and 18 that project aboveand below the radially outer bearing portion 14 for engaging in indentsin confronting surfaces 20 a and 22 a of the adjacent upper and lowervertebrae 20 and 22, respectively, for seating the implant 10 therein.In the implant device 10, the central bearing 12 can be in the form of agenerally spherical ball such that the surface portions 16 and 18 arepart of the outer spherical surface 24 thereof. The annular bearingportion 14 has a generally ring-shaped (e.g. circular, oval orellipsoidal) body 17 that includes an arcuate inner side surface 26extending about the opening 15 that faces the ball bearing 12 havinggenerally the same radius of curvature as that of the spherical ballsurface 24 so that the ball bearing 12 can substantially freely rotatein the small, concave indents, divots or depressions 27 and 29 formed inthe vertebrae confronting surfaces in which the ball 12 seats, asdescribed above and shown in FIGS. 2A-2D. At the same time, thevertebral surfaces 20 a and 22 a, and particularly the concave indents27 and 29 formed therein can readily slide over the ball surface 24. Theconfiguration and rotation of the ball bearing 12 allows the spinevertebrae 20 and 22 to substantially undergo the normal range ofbiomechanical movement such as when the patient is twisting their backand/or bending it in various directions.

When the implanted disc 10 undergoes compressive loading, the outerbearing 14, and in particular the upper and lower surface portions 28and 30 thereof will substantially maintain the effective spacing betweenthe vertebrae 20 and 22. Thus, in the present artificial disc device 10,the outer ring bearing body 17 shares the loading with the ball bearingbody 13 created between the dynamically moving vertebrae 20 and 22 so asto avoid subsidence problems as occurred with prior ball bearing-typedevices. Accordingly, in the disc 10, the outer bearing 14 generallywill not allow the end plates to subside around the ball bearing 12.

As shown, the curvature of the upper and lower surface portions 28 and30 of the outer bearing body 17 is more gradual than that of the arcsurface portions 16 and 18 of the central ball bearing body 13 toprovide it with a doughnut-type configuration. Accordingly, in thedevice 10, the surface portions 28 and 30 are part of a substantiallycontinuously curved outer ring bearing surface 32 such that they curvearound the radially outermost point 29 of the outer bearing body 17 toform an outwardly projecting convex configuration 29 for the outersurface 32 of the annular bearing 14. As such, the surface portions 16and 18 extend to their greatest spacing at the central section 17 aadjacent the central opening 15 of the bearing body 17. At the thickestsection 17 a, the spacing of the surface portions 16 and 18 is less thanthe diameter of the ball bearing 12 so that the surface portions 16 and18 protrude from the opening 15 to extend above and below the respectiveouter bearing surface portions 28 and 30 for engaging in the concavedepressions 27 and 29. The gradual curvature of the surface portions 28and 30 allows the ring bearing 14 to better conform to the generalconcavity of the vertebral surfaces 20 a and 22 a including any attachedend plates over time. By way of example and not limitation, the ballbearing diameter can be approximately between 6-18 mm and the maximumthickness of the outer bearing section 17 a can be approximately 16 mm.Manifestly, these sizes are to be tailored according to the anatomy ofthe patient being treated.

Referring to FIGS. 1F-1J, artificial disc device 10 a is depicted whichhas a slightly modified wedged or bulged configuration for correspondingouter bearing body 14′ thereof. More particularly, as can be seen in thecross-sectional view of FIG. 1J, the outer bearing body 14′ has athickened section 17 a and thinner section 17 b as measured between thecorresponding upper and lower surface portions 28 a and 30 a with thesesections being generally diametrically opposite each other with a smoothtransition therebetween. Since the confronting vertical surfaces 20 aand 22 a will normally be in a non-parallel orientation relative to eachother, the section 17 a of the disc device 10 a will better conform tothe area between the surfaces 20 a and 22 a that are spaced further fromeach other with the section 17 b fitting better in the more confined,closely spaced area between the vertebrae surfaces 20 a and 22 aallowing the implant device 10 a to be tightly fit or wedged between thevertebrae 20 and 22.

With the vertebrae 20 and 22 exerting compressive loading on theartificial disc device 10, the projecting surface portions 16 and 18 ofthe center ball bearing 12 will securely engage in the indented recesses27 and 29 in the confronting vertebral surfaces 20 a and 22 a forseating the ball bearing 12 therein. As the spine moves causing relativeshifting of the vertebrae 20 and 22 about the ball bearing 12 with itfreely rotating in the recesses 27 and 29 as necessary, further loadingis exerted on the device 10, with the surface portions 28 and 30 of theouter annular bearing 14 being effective to share with the ball bearing12 the compressive loading that is generated between the vertebrae 20and 22, and which further can act as a shock absorber for the highimpact load bearing that may be needed between the vertebrae 20 and 22,such as described hereinafter. In this manner, the present artificialdisc device 10 resists both migration by the seating of the central ballbearing 12 as well as avoiding subsidence problems by providing loadbearing which is well distributed across a large radially extendingsurface area of the device 10 as by the device upper surfaces 16 and 28and lower surfaces 18 and 30. For example, the distance from the centralaxis 19 of the ring bearing 14 extending through the opening 15 to theouter end 29 can be approximately 12 mm.

While other material selections are possible, it is presentlycontemplated that the inner ball bearing 12 preferably will be of aharder material than the outer bearing 14 so that the harder ball 12 ismore apt to maintain its conformity with and thus stay seated in theindents 27 and 29 in the surfaces 20 a and 22 a. In this regard, theball 12 can be of a biocompatible material including titanium ormetallic material such as stainless steel, while the ring bearing 14 canbe of a material of a lower modulus of elasticity such as plasticmaterial, e.g polyethylene, so as to have some resilience undercompressive loading forces.

With a plastic outer bearing 14, a support hoop 34 of a harder materialthan that of the outer bearing 14 such as of metal material similar tothat of the ball bearing 12 can be embedded therein. Generally, thehardness of the ball bearing 12 and the hoop 34 will both be greaterthan the outer bearing 14, although they may not be the same as eachother. For example, the hoop 34 can be of a hard metal material whereasthe center bearing 12 can have a hardness similar to the human bone. Tothis end, the plastic outer bearing 14 can be a molded component of theartificial disc device 10. As such, the metal support hoop 34 can bemolded in situ in the outer ring bearing 14. The support hoop 34 servesas a compression limiter to resist deformation of the resilient plasticring bearing 14 due to the compressive loading generated between thevertebrae 20 and 22 so that it is better able to maintain itsconfiguration despite the stresses exerted thereon. In addition, thehoop 34 also resists shear forces generated by spinal movements forreducing such forces in the resilient material of the outer bearing 14.

Alternatively, outer bearing body 17 can have an inner core portion thatis of different and softer material than that of the harder outerportion so that the annular bearing 14 has improved shock absorbingproperties for high force impacts on the artificial disc 10 with theharder outer layer minimizing wear on the bearing 14. For example, thewear layer can be of hard polyethylene material with the inner cushionmaterial of the bearing body 17 being of a softer polymeric orelastomeric material. In another alternative, the body 17 can include ahollowed inner portion that is filled with liquid or gel or otherelastic material, e.g. Hydrogel and/or polyurethane, for shockabsorption purposes.

FIGS. 3A-3D and 5A-5E are directed to alternative artificial discdevices 36 and 38, respectively. The disc devices 36 and 38 are ofsimilar construction as each include a central bearing portion 39 formedfrom two opposing shells 40 and 42 having a generally dome-shapedconfiguration riding on a central or inner, spherical ball bearingportion 44 that can be formed integrally with a body 43 of radiallyenlarged bearing 46 including outer bearing portion 45 thereof. Oppositeupper and lower annular arcuate spaces 43a and 43b are formed in thebody 43 separating the bearing portions 44 and 45 by a distance greaterthan the thickness of the shells 40 and 42 so that respective shell endportions 47 and 49 fit therein allowing the dome shells 40 and 42 toslide on the ball bearing portion 44.

Other differences in the construction of the bearing 46 of the devices36 and 38 relates to the plan configuration of the outer bearing portion45. The devices 36 and 38 have their bearing portion 45 provided with apair of lobe sections 48 and 50 that extend in a continuously curvedpath about the majority of their peripheries until the lobe perimetersmeet at their juncture formed at a recessed area 52 therebetween. Inthis manner, the plan shape of the lobed bearing 46 more closelyapproximates that of the vertebrae 20 and 22 between which the devices36 and 38 are implanted. Ring bearing 14 could be provided with asimilar lobed plan configuration. Manifestly, the outer bearings 14 and46 can be formed with other configuration, e.g. oval in plan, so as tobe more closely match that of the intervertebral space in which they areto be implanted.

Another difference resides in the configurations of load bearing surfaceportions 54 and 56 of the bearing 46 generally corresponding to the loadbearing surface portions 28 and 30 of the bearing 14. In contrast to thecurvature of the surfaces 28 and 30 of the ring bearing 14, the surfaces54 and 56 are shown as having a generally flat, parallel configurationso that the bearing body 43 has more of a disc or plate-likeconfiguration. Generally, however, some curvature on these bearingssurfaces 54 and 56 will be desirable although perhaps modified from thatshown for bearing surfaces 28 and 30 for the implant 10. The surfaces 54and 56 are provided with a spacing smaller than that of the diameter ofthe central bearing portion 44 and thus of the central bearing assembly39 with the dome shells 40 and 42 thereon so that they project above andbelow the respective surfaces 54 and 56. In this manner, the dome shells40 and 42 are able to seat in indents 27 and 29 in the vertebralsurfaces 20 a and 22 a like the bearing ball surface portion 16 and 18.To this end, the shells 40 and 42 can be of harder material than that ofthe bearing body 43, and particularly the ball bearing portion 44thereof. Accordingly, similar to the ball bearing 12, the dome shells 40and 42 can be of a ceramic material or a stainless-steel metal, titaniumor alloys thereof, whereas the ring bearing 46 is preferably of aplastic or polymer material such as polyethylene to provide it withstiffness and resiliency under compressive loading. The bearing 46 couldalso be of like material to that of the dome shells 40 and 42 for higherload bearing capacity.

The dome shells 40 and 42 are sized relative to the spherical bearingportion 44 such that there are gap spacings 57 between peripheral endedges 58 and 60 of the respective shells 40 and 42 at their largestdiameters and web wall 62 in the bearing 46, as best seen in the crosssectional views of FIGS. 3D and 5D. Accordingly, the diameter across theend edges 58 and 60 of the dome shells is less than the diameter of theball bearing portion 44. In use, the dome shells 40 and 42 can slide totake up these spaces 57.

The web wall 62 extends laterally or radially and centrally from theball bearing portion 44 to the annular load bearing portion 45 thatextends about the ball bearing portion 44 on which the shells 40 and 42ride. The circumferential web wall 62 extends radially for a sufficientdistance, such that the outer bearing portion 45 is spaced from the ballbearing portion 44 to provide recesses 43 a and 43 b large enough toallow the dome edges 58 and 60 to slide into engagement with the webwall 62 without encountering interference from the annular load bearingportion 45 of the bearing 46.

In the device 38, the annular bearing portion 45 includes a radiallyinner surface 51 that extends generally axially or tangentially to outerspherical surface 44 a of the inner bearing portion 44, albeit spacedslightly therefrom via web wall 62. In this manner, the correspondingspaces 43 a and 43 b in the body 43 of the device 38 are enlarged overthose in device 36 such that overhanging portions of the bearing portion45 that can be compressed against the dome shell portions 47 and 49 andpotentially cause binding in the spaces 43 a and 43 b are avoided.

With the above-described construction, the artificial disc devices 36and 38 have a bi-polar construction in that relative movement betweenthe vertebrae 20 and 22 and the dome shells 40 and 42 can occur alongwith relative movement between the dome shells 40 and 42 and the ballbearing portion 44. Generally, the smooth surface interface betweeninner surfaces 40 a and 42 a of the respective shells 40 and 42 and theouter surface 44 a of the ball bearing portion 44 will have a lowercoefficient of friction therebetween than that between outer surfaces 40b and 42 b of the respective shells 40 and 42 and the indents 27 and 29in the vertebrae surfaces 20 a and 22 a. Thus, there will be somedifferential shifting that can occur with the moving components of thedevices 36 and 38 such that generally the domes 40 and 42 will morereadily shift along the ball bearing portion 44 prior to shifting of thedome shells 40 and 42 with respect to the vertebrae 20 and 22. Suchdifferential articulation keeps wear between the higher coefficient offriction surfaces to a minimum as sliding can preferentially occurbetween the smooth inner arcuate surfaces 40 a and 42 a of therespective shells 40 and 42 and the outer surface 44 a of the ballbearing portion 44. Alternatively, if the coefficient of friction islower between the vertebrae surface concave indents 27 and 29 and theshell outer surfaces 40 b and 42 b, then of course shifting willpreferentially occur at this interface of the disc devices 36 and 38keeping wear at the higher friction interface between the shell innersurfaces 40 a and 42 a and ball surface 44 a to a minimum. Of course, asthe spine is undergoing various dynamic forces during the movements itis required to undertake, oftentimes both interfaces of the bi-polardevices 36 and 38 will be shifting simultaneously to provide the spinewith the necessary biomechanics while also keeping undue wear on thedisc devices 36 and 38 to a minimum.

FIGS. 4A-4D illustrate the lobed artificial disc device 36 implantedbetween the adjacent upper and lower vertebrae 20 and 22. In the view ofFIG. 4C, the disc device 36 is employed with the annulus 65 kept intact,and in the other view, the annulus 65 is removed with the disc device 36implanted. To maintain the annulus 65, the disc device 36 is insertedthrough an incision in the annulus 65 which may be repaired once thedevice 36 is implanted. In this instance, the device 36 reinflates theannulus 65 keeping it taut and relieves the compressive loading on theannulus 65. The other artificial disc devices described herein can beemployed in a like manner to that of device 36.

The annular load bearing body portion 45 of the device 36 has an outerperipheral surface 66 (FIG. 3C) with a generally convex configurationsimilar to the convex curved configuration at the corresponding radiallyouter location of the outer annular bearing 14. In contrast, thecorresponding surface 68 of the load bearing portion 46 of the device 38shown in FIG. 5C has a grooved or concave configuration to form thinnedupper and lower flange rims 70 and 72 thereof. The above-describedconstruction for the bearing 46 as shown in FIG. 5C provides it withgreater flexibility as the flanges 70 and 72 are better able to flextoward each other under compressive loading and thus are optimized froma shock absorption standpoint. In particular, by having the flanges 70and 72 extending around the entire circumference of the bearing 46,compressive loads taken locally by the bearing 46 such as due to bendingof the spine in a particular direction will cause the portions of theflanges 70 and 72 thereat to flex toward each other about the concaveperipheral surface 68 while the remainder of the disc 46 including theunloaded portions of the flanges 70 and 72 will remain substantiallyundeformed. Once this loading is removed, the bent portion of theflanges 70 and 72 can resiliently flex back to their illustratedsubstantially undeformed configuration. In this manner, the flanges 70and 72 better permit directional deformation of the bearing 46.

Optionally, upper and lower annular layers including the flanges 70 and72 can be provided of harder material than a more flexible core materialof the bearing body 43 for optimized wear resistance at the interfaceswith the vertebral surfaces 20 a and 22 a and also for improved shockabsorbing properties for the device 38 a. For instance, the wear layerscan be of hard polyethylene while the core of the body 43 would be ofmore flexible, e.g. elastomeric, cushioning material.

Referring next to FIGS. 6A-6F, another artificial disc device 74 inaccordance with the invention is illustrated. The artificial disc device74 is similar to the device 10 of FIGS. 1A-1E in that it includes acentral ball bearing 76 such as of ceramic material or stainless steelor titanium metal and alloys thereof or having carbon fiber or otherbiocompatible materials therein and including projecting arc surfaceportions 78 and 80 for seating in the indents 27 and 29 in the vertebralsurfaces 20 a and 22 a, as previously described. The device 74 ismodified over device 10 in that rather than having a doughnut shapedbearing 14, the device 74 includes a pair of annular plates or discs 82and 84 such as of a metallic material vertically spaced along centralaxis 86 that extends through the central openings 88 and 90 formed inthe respective discs 82 and 84 in which the ball bearing 76 is received.As shown, the disc openings 88 and 90 are of a maximum diameter that isslightly less than that of the diameter of the ball bearing 76 such thatwhen the arcuate surfaces 83 and 85 about the openings 88 and 90 are inclose fit with the outer ball surface 92 and the discs 82 and 84 are ina generally parallel orientation, the discs 82 and 84 will be spaced bya gap 94 therebetween.

With the device 74 loaded and the confronting vertebral surfaces 20 aand 22 a engaging and pushing on the discs 82 and 84, they will shiftand pivot relative to each other and axis 86 closing the gap 94 atcertain locations thereabout and opening it at others. As such, it isthe upper surface 82 a and lower surface 84 a of the respective upperand lower discs 82 and 84 that are the major load bearing surfaces forthe device 74. As shown, these surfaces 82 a and 84 a can be contouredso that the respective discs become thicker as extending from theperiphery toward the respective openings 88 and 90 of the discs 82 and84.

In an alternative form, a resilient and flexible cushioning material 95can be attached between the discs 82 and 84. The material 95 will keepthe unloaded discs 82 and 84 in the illustrated, generally parallelorientation, but also allow them to undergo relative shifting undercompressive loading. In this regard, the material 95 is selected so thatit can resiliently expand and contract as the discs 82 and 84 shift andtilt or pivot with respect to each other. Alternatively, the unloadeddiscs 82 and 84 could be canted to a non-parallel orientation relativeto each other to provide the disc device 74 with a wedged configurationsimilar to the previously-described device 10 a.

Accordingly and as described above, as the spine and particularly thevertebrae 20 and 22 exert compressive loading on the discs 82 and 84,they can shift relative to one another so they are better able toconform to the position of the vertebrae 20 and 22 as they shift withspine movement. For example, if the patient bends anteriorly, the upperdisc 82 can tilt relative to the axis 86 so the gap spacing 94 betweenthe discs 82 and 84 can be greater at the rear portion than at theforward portions thereof. In a like manner, if the patient bends theirspine posteriorly, then the upper disc 82 can pivot about axis 86 suchthat the gap spacing 94 can be greater at the forward portions relativeto the spacing at the rear portions. In each instance described above,there will usually be some tilting of the lower disc 84 as well althoughnot to the same degree as that of the upper disc 82 so that theirtilting movements relative to the axis 86 generally will correspond tothat of the upper and lower vertebrae 20 and 22 and the surfaces 20 aand 22 a thereof relative to the axis of the spine.

The discs 82 and 84 can have a plan configuration akin to that of thelobed bearing 46, or alternatively they can be oval or ellipsoidal. Asshown in the plan view of FIG. 6D, the configuration of the discs 82 and84 includes a larger recessed or concave area 94 as compared with thecorresponding recess area 52 of the ring bearing 46. Further, thecurvature of the remainder of the disc periphery 96 varies from aconvexly curved portion 98 opposite the recessed area 94 to straighteropposite sides 100 and 102 on either side of the recessed area 94.

Turning to FIGS. 7A-7C, an alternative construction for the centralbearing 39 is shown. In this version, a pair of opposing domes 104 and106 are provided which ride on an inner bearing portion 107 similar topreviously-described ball bearing portion 44, albeit modified toaccommodate the projecting post 108 and hub 110, which are describedbelow.

The hub 110 can have a recess 112 which can engage against the distalcurved end 114 of the post 108 to resist the compressive forces thatotherwise would push the dome shells 104 and 106 further toward eachother. More particularly, the dome shell 104 has an end edge 116 and thepost 108 extends centrally from the shell 104 along axis 118 so that itprojects beyond the edge 116. Likewise, the shell 106 includes an endedge 120 beyond which the hub 110 can project along the central axis 118so that it is in alignment with the post 108. The post 108 and hub 110have their respective sizes coordinated so that they define a limit atwhich spacing 122 between the dome shells 104 and 106 cannot be exceededwith the end edges 116 and 120 extending generally parallel to eachother. In this manner, unlike the previously described central bearingassemblies 39 that rely on the stiffness or resilience of the polymericspherical bearing portion 44 to resist compression of the dome shells 40and 42, the dome shells 104 and 106 which are preferably of a hardermaterial such as metal employ the cooperating integral post 108 and hub110 for limiting the maximum compression that can occur therebetween. Asis apparent, under normal conditions, the post 108 and hub 110 will bespaced or only lightly engaged so that they do not bear the loadsgenerated between the vertebrae 20 and 22.

As mentioned above and referencing FIGS. 8A-8E, the central or innerbearing portion 107 is modified so that the post 108 and hub 110 canproject therethrough. As seen in the cross-sectional view of FIG. 8E,the bearing portion 107 has an axial through opening 124 havingreversely configured upper and lower frustoconical surface portions 124a and 124 b into which the post 108 and hub 110 extend, respectively.The surface portions 124 a and 124 b taper from the largest size of theopening 124 at the dome surfaces to the smallest size of the opening 124at the center of the ball bearing portion 107. This provides the domes104 and 106 with freedom of movement about the ball bearing portion 107allowing the post 108 and hub 110 to rock back and forth until the domeends 116 and 120 engage the web wall 62 without encounteringinterference from the surface portions 124 a and 124 b, respectively.

In FIG. 9, a further variation of the central bearing assembly shown inFIGS. 8A-8E is illustrated. In this version of an artificial disc device125, instead of having the apertured central bearing portion 107 that isintegrally connected to the web wall 62, upper and lower inner bearingrings 126 and 128 are provided supported by an inner extension 130 ofthe web wall 62 that extends therebetween. The rings 126 and 128 eachhave an outer arcuate bearing surface 126 a and 128 a on which the domeshells 104 and 106 ride. The rings 126 and 128 can also translate alongthe web wall 62 to provide for lateral movement of either or both domeshells 104 and 106 during articulation of the spine such as when thepatient bends their spine in flexion or extension. In this manner, thedevice 125 provides for an even greater range of motion than thepreviously described devices as there are now three shifting interfacesincluding the innermost interface between the rings 126 and 128 and webwall 62 enabling the dome shells 104 and 106 to reciprocate therealong.At the same time, the shells 104 and 106 may be rotating in the indents27 and 29 and rotating about the rings surfaces 126 a and 128 a, such asin the previously-described devices. For wear resistance, the rings 126and 128 can be of a hard polyethylene material while the web wall 62 ispreferably of a more flexible or pliant material for shock absorptionpurposes. For sliding of the rings 126 and 128 on the web wall 62, itcan be coated with a harder material or have washers of metallic or alike hardness material attached to upper and lower surfaces thereof toreduce the friction coefficient with the rings 126 and 128 slidingthereon.

Referring next to FIGS. 10A-10H, an alternative artificial disc implantdevice 132 is illustrated in which there is an enlarged, central bearingmember 134 and an outer bearing member 136 which share the compressiveloads generated between the vertebrae 20 and 22 during typical spinemovements. The central bearing member 134 has a post body 138 that isaxially elongated such that upper and lower arcuate bearing surfaces 140and 142 generally extend beyond corresponding upper and lower bearingsurfaces 144 and 146 formed on annular body 148 of the outer bearingmember 136, similar to the previously-described disc implants herein.

The outer bearing body 148 has a central through opening 150 that isbounded by a cylindrical inner surface 152 in close confronting relationto outer side surface 154 on the post body 138. To provide optimizedcontrolled resiliency of the shape retentive bearing body 148, throughapertures 156 can be formed at selected locations extending axiallytherethrough, as shown in FIG. 10D. These apertures 156 provide anincrease in the normal compressibility or coefficient of restitution ofthe material, e.g. plastic, of the bearing body 148. Based on theposition, pattern and/or density of the through apertures 156, theflexibility or compressibility of the body 148 can be increased ordecreased in a localized fashion. Of course, these apertures 156 couldbe employed in the other disc implants and specifically the bodies ofthe outer bearings thereof in a like fashion. Similarly, thepreviously-described liquid or gel material, e.g. Hydrogel, used in theouter bearing body 17 could also be provided in the apertures 156 sothat they do not extend all the way through the body 138 and insteadserve as chambers for the visco-elastic material therein to varycompressibility of the body 148.

For instance and as shown in the plan view of FIG. 10A, the frequency ofthe apertures 156 can be increased in a radially outward direction fromthe central opening 150 to the periphery of the bearing body 148 so thatin a like fashion the body 148 can be more easily compressed toward theperiphery thereof. Alternatively, the size or diameter of the holes 156can vary such as by having, for example, smaller size apertures 156 acloser to the central opening 150, larger size apertures 156 b closestto the radially outer periphery of the body 148, with apertures 15 6 chaving sizes intermediate those of apertures 156 a and 156 b generallydisposed therebetween, as shown in FIG. 10E. As is apparent, byselective spacing and/or sizing of the aperture 156, the bearing body148 can be made to be more or less flexibly resilient at preciselocations thereabout. In this manner, the bearing body 148 can bestiffer in locations where load bearing is more critical and morecompressible at positions were shock absorption is more important. It isalso anticipated that the apertures 156 will provide stress relief forthe load bearing body 148 so as to increase the life thereof.

As seen in the cross-sectional views of FIGS. 10D and 10H, the post body138 preferably is provided with a recess in its surface 154 such asannular groove 139 formed approximately midway along the body lengthbetween the bearing surfaces 140 and 142 thereof. By way of this groove139, there is a gap 155 that is formed between the confronting bearingsurfaces 152 and 154. When the resilient body 148 of the outer bearing136 is compressed, the gap 155 provides space into which the resilientmaterial of the body 148 can deform and expand laterally.

An alternative disc device 175 is shown in FIGS. 11A-11D having upperand lower disc plate members 176 and 178 with there being a load bearingmember 180 therebetween. However, unlike prior devices, the device 175like other devices described herein allows for relative movement betweenthe vertebrae 20 and 22 and the respective vertebral engaging members176 and 178. The plate members 176 and 178 have arcuate vertebralengaging surfaces 182 and 184 formed thereon having a gradual curvatureor slope extending from the outer periphery up toward central axis 186of the device 175. As the surfaces 182 and 184 approach the axis 186they begin to extend more axially than radially to form centerprojections 188 and 190. These projections 188 and 190 are shown inFIGS. 11C and 11D as being provided with a tip or point end 191 and 193for piercing into the vertebral bone locating the device 175 implantedbetween the vertebrae 20 and 22 although they also could simply becurved or sloped as shown in FIGS. 11A and 11B to serve the samelocating function similar to the center arcuate surface portions ofpreviously-described devices.

Accordingly, the surfaces 182 and 184 include radially extending bearingsurface portions 182 a and 184 a that extend radially along therespective facing vertebral surfaces and central, axially extendingbearings surface portions 182 b and 184 b that serve to locate thedevice 175 while also allowing relative sliding rotation of thevertebrae 20 and 22 thereabout and specifically 360° about device axis186 since the plate members 176 and 178 are not fixed to the respectivevertebrae 20 and 22. The center surface portion 182 b and 184 b onlyresist lateral sliding of the plates 176 and 178 by fitting incorrespondingly shaped recesses or openings in the vertebral facingsurfaces 20 and 22 a and otherwise are not fixed or fastened thereto.

As shown, the member 180 has a spherical ball configuration. The plates176 and 178 have arcuate recessed surfaces 192 and 194 opposite theirsurfaces 182 and 184 and in which the ball member 180 seats. The ballmember 180 can be of a harder material, e.g. steel, than the softer discplate members 176 and 178. The materials for the members 176-180 ispreferably selected for low frictional resistance to relative slidingmovement therebetween to allow rotation of the members 176-180 such aswhen the spine is twisted and to allow relative sliding between theplate members 176 and 178 and ball 180 such as when the spine is bent inflexion and extension with the plates 176 and 178 pivoting with respectto each other. In this manner, the device 175 is bi-polar since thereare two shifting interfaces thereof, i.e. between the plates 176 and 178and the vertebrae 20 and 22 and between the ball 180 and the plates 176and 178.

FIGS. 11E-11H are views of another device 175′ similarly constructed todevice 175 including upper and lower plates 176 and 178 with a ballbearing 180 therebetween. The device 175′ also includes an annularmember 196 that extends about the ball bearing 180 with the plates 176and 178 engaged against upper and lower surfaces 196 a and 196 bthereof. The annular member 196 acts as a shock absorber and can beformed of an elastomeric or other resilient material.

As is apparent, the various forms of artificial disc devices disclosedherein rely on both a center bearing portion and an outer, annularbearing portion extending about the center bearing portion to provideimplants that resist migration without relying on disc fixing mechanismssuch as intrusive bone fasteners, clamps and the like while alsoavoiding subsidence problems about the center bearing portion. To thisend, the upper and lower arcuate surfaces of the center bearing orbearing portion or bearing assembly seat in correspondingly shapedrecesses 27 and 29 in the vertebral surfaces 20 a and 22 a to locate theartificial disc device between the vertebrae 20 and 22. The interfacebetween the center bearing surface portions and the recesses 27 and 29is preferably a sliding one, i.e. not fixed, to substantially providethe vertebrae with their normal range of motion relative to each otherwith the discs implanted therebetween. And because of the enlarged axialspacing of the surface portions relative to the outer bearing portion,be they formed on separate components such as the dome shells or on asingle part such as center ball or post bearings, the convex curvatureof the center surface portions seated in the concave recesses providesresistance against migration or lateral shifting of the device out frombetween the vertebrae.

Extending about these axially projecting center bearing surface portionsare outer bearing surface portions that also extend radially outwardlytherefrom, generally with a more gradual curvature or with a flatconfiguration. As shown, the outer bearing surface portions extend sothat their radial outer ends are close to the periphery of therespective vertebral bodies thereabove and therebelow. Accordingly, theupper outer bearing surface portion is generally lower than the axiallyprojecting upper center bearing surface portion, and they form ajuncture at which the direction in which the surface portions of thedisc device for engaging the vertebrae changes or transitions from oneextending more axially to one extending more radially. This juncture isa direction transition area and does not necessarily mean that thesurface portions are joined thereat, such as can be seen with thepreviously-described ball bearing 12 and ring bearing 14 which areseparate components with the ring bearing 14 extending annularly aroundthe ball bearing 12 so as to allow for relative movement therebetween.Similarly, the lower outer bearing surface portion is generally higherthan the axially projecting lower center bearing surface portion, and attheir juncture the direction of the vertebral engagement surface portionof the device also changes as described above with respect to the uppervertebral engagement surface portions. In this manner, these radiallyextending outer surface portions limit the ability of the vertebrae ortheir attached end plates to subside around the center bearing. If thereis any subsidence, its extent is limited by the axial spacing of theupper and lower outer bearing surface portions. In other words, in thearea taken up by the artificial disc, the spacing of the upper and lowervertebrae can not be less than the spacing between the outer bearingsurface portions, thereby limiting subsidence problems accordingly.

In another version of a disc device 200 in accordance with the aboveprinciples, upper and lower arcuate center bearing surface portions 202and 204 that are convexly curved are provided for locating the device200 between adjacent vertebrae in corresponding arcuate concave recessesformed therein. Upper and lower outer bearing surface portions 206 and208 extend annularly about respective center bearing surface portions202 and 204 and limit subsidence between the vertebrae about the centerbearing portion 210 of the device 200. The upper surface portions 202and 206 are formed integrally on an upper plate member 212, and thelower surface portions 204 and 208 are formed integrally on a lowerplate member 214. The plate members 212 and 214 can be of a hardbiocompatible material such as titanium coated with a pyroletic carbon.Like previously-described discs, the center bearing surface portions 202and 204 are spaced by an axially greater distance than the outer bearingsurface portions 206 and 208, and they have a smaller radius ofcurvature than the more gradual curvature of the surface portions 206and 208. As such, as the vertebral engaging surface portions extend awayfrom the disc axis 216, there is upper and lower junctures 218 and 220where the direction and configuration of the surface portions undergo anabrupt change from one where the surface portion 202 or 204 extends moreaxially versus one where the surface portion 206 or 208 extends moreradially to provide subsidence resistance about the center bearing 210.To this end, the plate members 212 and 214 include respective small,axial projections 213 and 215 that are centrally disposed relative todisc axis 216 and on which the respective center bearing surfaceportions 202 and 204 are formed.

As part of annular, outer bearing portion or assembly 222 extendingabout the center bearing assembly 210, an annular load bearing portionor member 224 is provided axially between the upper and lower bearingplates 212 and 214. The member 224 is preferably of a resilient materialsuch as an elastomeric or resiliently compressible polymeric material,e.g. polyurethane and silicone combination, or a hydrogel material, fortaking loads that are generated between the vertebrae during normalspinal movements. The annular member 224 has an axial thickness sized tomaintain the plates 212 and 214 spaced axially by an anatomicallycorrect distance from each other for engaging the vertebrae and keepingthem properly spaced. At the same time, the resilient material of theload bearing member 224 allows the plates 212 and 214 to shift ordeflect relative to each other during dynamic relative movements of thespine vertebrae 20 and 22 such as when the spine is being twisted andbent as in flexion or extension movements. For example, at one end ofthe disc 200, the plates 212 and 214 may be pivoting toward each othercompressing the member 224 therebetween while at a generallydiametrically opposite end the plates 212 and 214 will pivot or shiftaway from each other allowing for expansion of the resilient material ofthe member 224 in this area between the plates 212 and 214.

The annular bearing member 224 can be a composite to include a harderlow friction wear coating on its upper and lower surfaces to allow thefacing lower and upper surfaces of the respective upper and lowerbearing plates 212 and 214 to minimize wear in this interface area suchas when compressional and/or torsional forces are applied therebetween.Alternatively, upper and lower annular washers or wear plates 226 and228 can be inserted in the interfaces between the upper bearing plate212 and the load bearing member 224 and the lower bearing plate 214 andthe load bearing member 224 to allow the plates 212 and 214 to have alow friction surface in engagement therewith.

The annular configuration of the load bearing member 224 of the outerbearing portion 222 forms an interior central space 230 in which abumper or plug member 232 is provided as part of the center bearingportion 210 of the device 200. The bumper member 232 fits somewhatloosely in the interior space 230 and is of a harder material having ahigher modulus of elasticity than the outer bearing member 224. Thus,the plug member 232 is operable during high impact loading on thevertebrae to keep the annular member 224 from deforming too much andoverloading. In normal loading conditions, there is a spacing betweenthe upper plate member 212 and the bumper member 232. The harder plugmember 232 allows the annular member 224 to be softer so that itscushioning function between the vertebrae can be maximized. At the sametime the material of the member 224 needs to be of sufficient stiffnessor resiliency so as to be substantially shape retentive for maintainingstability between the vertebrae over millions of cycles and withoutexperiencing undesirable material creep or plastic deformation due tothe heavy loading it will undergo.

As can be seen in FIGS. 12A and 12C, the plates 212 and 214 haverespective arcuate projections 234 and 236 that extend toward each otherin the interior space 230. The plug member 232 has upper and lowerarcuate recesses 238 and 240 concavely configured to mate with theconvex configuration of the arcuate projections 234 and 236,respectively. The relative sizing of the space 230 and the plug member232 therein is such that when the plug member 232 rests on the lowerplate 214 via seating of the projection 236 in the recess 240, therewill be an axial gap 242 between the plug 232 and the upper plate 212and specifically the respective surface 238 and projection 234 thereof.Accordingly, the annular member 224 has a greater axial thickness thanthe plug member 232. The space 230 has a larger diameter than the plugmember 232 so that there is a generally lateral space between the innersurface 224 a of the annular member 224 and the plug member 232 allowingfor lateral deformation of the resilient member 224 when loaded. Whenthe vertebrae are overloaded such as due to shock or high impact loads,the normal loading ring member 224 is compressed taking up the axial gap242 such that the projection 234 engages the harder plug member 232 inthe recess 238 thereof. In this manner, further compression andoverloading of the resilient member 224 is avoided. Also, engagement ofthe projections 234 and 236 in their recesses 238 and 240 resistsrelative lateral shifting between the plates 212 and 214, and theannular member 222.

It is also contemplated that the annular member 224 and plug member 232could be integrally formed with one another, although having the members224 and 232 as separate components is the preferred form for the presentdisc device 200.

As best seen in FIG. 12C, the arcuate projections 234 and 236 are largerthan the respective arcuate surface portions 202 and 204. Theprojections 234 and 236 are centrally disposed relative to axis 216 andextend radially for a greater distance on either side of axis 216 thando the arcuate surface portions 202 and 204 so that there is a greaterbearing surface interface between the plate projections 234 and 236 andthe plug member 232 than between the locating surface portions 202 and204 and the vertebrae. As such, when the plug member 232 is loaded, itprovides relatively large bearing surfaces for the plates 212 and 214,and also allows for pivoting between the plates 212 and 214 with theplate central projections 234 and 236 sliding in respective recesses 238and 240 and with compression and expansion of generally diametricallyopposed portions of the member 224 depending on the exact location ofthe loads placed on the device 200. Alternatively, the surface portions202 and 204 can be similarly sized to the projections 234 and 236 oreven larger for maximizing the bearing surface area they provide betweenthe device and the vertebrae.

In FIGS. 12D and 12E, the disc device 200 is shown with modificationsincluding an annular sheath 244 that extends about the outer peripheryof the device 200. The sheath 244 includes upper and lower lips 246 and248 and that grip around and onto the upper and lower surfaces 206 and208, respectively, of the outer bearing assembly 222 to hold the device200 in its assembled form for implantation. Alternatively, a bagcompletely encasing the device 200 could be employed. Also, a retainingstructure, generally designated 250, can be provided between the plates212 and 214 and the annular member 224 for resisting relative lateralshifting between the plates 212 and 214, and the member 222, as well asresisting relative rotational shifting therebetween for keeping thesedisc components aligned. Projecting posts 252 and 254 can project downfrom the underside of the upper plate 212, and posts 256 and 258 canproject up from the upper side of the lower plate 214. Correspondingaperture pairs 260 and 262 can be formed in the upper and lower surfacesof the annular bearing member 224 for receiving the respective postpairs 252, 254 and 256, 258 therein, as can be seen in FIG. 12E.Alternatively, the location of the posts and apertures could bereversed. In another alternative form of retaining structure 250, upperand lower annular grooves 261 (upper groove 261 shown in ghost in FIG.12D) can be formed in upper and lower surfaces of the annular bearingmember 224 for receipt of corresponding upper and lower raised ridgesformed on the resilient annular member 224. Since the plan shape orconfiguration of the plates 212 and 214 and member 224 are other thancircular, it is desirable for the ridges and grooves to be similarlyconfigured so that relative rotational sliding as well as translationalor lateral sliding between these components is resisted. Again, thecomponents on which the cooperating grooves and ridges are formed can bereversed from that described above.

Instead of the posts/recess or groove/ridge structure 250, the structure250 can be provided at the periphery of the device 200, as shown in FIG.12F. The upper plate 212 includes a downwardly extending peripheral lipprojection 263, and the lower plate 214 includes an upwardly extendingperipheral lip projection 264. The resilient member 224 is provided withperipheral grooves 266 and 268 in which the lips 262 and 264 extend soas to restrain the member 224 against lateral and rotational shiftingrelative to the plates 212 and 214.

FIGS. 12G-12I show device 200 modified to include upper and lowerrecessed channels 270 formed in the upper and lower surfaces of theannular member 224 that extend from the inner, axially extending surface224 a to the outer peripheral surface 224 b of the member 224 to formopenings at each surface. In this way, the interior space 230 in whichthe plug member 232 is received communicates with the space external tothe device 200 via the flowpaths provided by the channels 270. Thus, thechannels 270 allow for fluid flow into and out from the device betweenthe plates 212 and 214 and the annular member 224. The channels 270 alsokeep vacuum conditions from developing in the space 230 as its volumecontinually varies with vertebral movements and thus the channels 270serve as a vacuum breaker for the device 200. The channels 270 can beprovided in a radial pattern so that there are several pairs of channels270 extending in radially opposite directions from the center space 230,as best seen in FIG. 12H.

While there have been illustrated and described particular embodimentsof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and it isintended in the appended claims to cover all those changes andmodifications which fall within the true spirit and scope of the presentinvention.

1. An artificial disc device for being implanted between upper and lowervertebrae, the artificial disc device comprising: upper and lowerbearing members having respective outer surfaces generally extending ina radial direction for projecting along and engaging against respectivesurfaces of the upper and lower vertebrae, and having respective innerarcuate surface portions; and a central bearing portion having upper andlower arcuate surface portions for bearing against the inner arcuatesurface portions of the upper and lower bearing members, respectively.2. The artificial disc device of claim 1 wherein the outer surfaces ofthe upper and lower bearing members engage slidingly with the respectivesurfaces of the upper and lower vertebrae.
 3. The artificial disc deviceof claim 1 wherein the outer surfaces of the upper and lower bearingmembers include surface features for limiting relative motion betweenthe upper and lower bearing members and the engaged surfaces of theupper and lower vertebrae.
 4. The artificial disc device of claim 1wherein the inner arcuate surface portions of the upper and lowerbearing members are convex, and the upper and lower arcuate surfaceportions of the central bearing portion are concave.
 5. The artificialdisc device of claim 1 wherein the central bearing portion includes atleast a resiliently deformable portion.
 6. The artificial disc device ofclaim 5 wherein the resiliently deformable portion is an elasticallycompressible portion.
 7. The artificial disc device of claim 6 whereinthe elastically compressible portion is generally deformed when theupper and lower vertebrae subject the artificial disc device toflexion-extension stresses.
 8. The artificial disc device of claim 6wherein the elastically compressible portion is generally deformed whenthe upper and lower vertebrae subject the artificial disc device toaxial stresses.
 9. The artificial disc device of claim 6 wherein theelastically compressible portion is at least in part axially alignedbetween the arcuate surface portions of the upper and lower bearingmembers.
 10. The artificial disc device of claim 1 wherein the upper andlower bearing members include inner radially extending surface portionspositioned radially outward from the inner arcuate surface portions. 11.The artificial disc device of claim 10 wherein the central bearingportion includes at least a resiliently compressible portion, and atleast a part of the resiliently compressible portion is axially alignedbetween portions of the inner radially extending surface portions of theupper and lower bearing members.
 12. The artificial disc device of claim11 wherein the resiliently compressible portion is annular.
 13. Theartificial disc of claim 11 further including an annular portionpositioned around radial edges of the upper and lower bearing membersfor restraining radial extension due to deformation of the resilientlycompressible portion.
 14. An artificial disc device for being implantedbetween upper and lower vertebrae, the artificial disc devicecomprising: upper and lower arcuate bearing members for engaging againstrespective facing surfaces of the upper and lower vertebrae; a centralbearing portion having an arcuate outer surface that supports themembers for sliding thereon to allow for relative movement between themembers and the central bearing portion during vertebral movements; andan annular outer bearing portion extending around the central bearingportion for load sharing with the bearing members.
 15. The artificialdisc device of claim 14 wherein the central bearing portion has agenerally spherical configuration and the members have a domeconfiguration for sliding on the spherical central bearing portion. 16.The artificial disc device of claim 15 wherein the spherical centralbearing portion has a predetermined diameter, and the dome members havea maximum diameter that is less than the predetermined diameter.
 17. Theartificial disc device of claim 14 including a web wall thatinterconnects and spaces the central and outer bearing portionslaterally from each other to keep interference between the slidingbearing members and the outer bearing portion to a minimum.
 18. Theartificial disc device of claim 14 wherein the annular outer bearingportion is of resilient material and has an outer peripheral surfaceincluding upper and lower flanges and a concave groove between theflanges to optimize flexibility of the flanges under compressiveloading.
 19. The artificial disc device of claim 14 wherein the outerbearing portion includes a core of cushioning material for shockabsorbtion and upper and lower bearing surfaces of hard material forwear resistance.
 20. The artificial disc device of claim 14 wherein thecentral bearing portion includes an axial through opening, and themembers include projections that extend in the through opening andcooperate to limit compression of the central bearing portion andshifting of the members axially toward each other.